With the recent developments of electrophotographic copying machines which can produce copies of various sizes at an increased speed, there has been an increasing demand for a photoreceptor to be used in these electrophotographic copying machines to have higher photosensitivity and longer working life.
Many proposals of so-called separate function type electrophotographic photoreceptors in which a plurality of elements respectively perform functions of a photoreceptor have been made in an attempt to improve various electrophotographic characteristics, such as charge retention, stability on repeated use, light response, spectral characteristics, and mechanical strength.
These electrophotographic photoreceptors are known to have disadvantages, such as (1) poor stability of image contrast against repeated use or environmental changes, (2) liability to cause image defects called white pepper, black pepper, roughness, pinholes, etc., and (3) insufficient durability due to low adhesive strength between a substrate and a photosensitive layer, causing separation of the photosensitive layer during use.
In order to eliminate these disadvantages, it has been proposed to provide a resin subbing layer between a substrate and a photosensitive layer. Resins known for the subbing layer include poly-p-xylene, casein, polyvinyl alcohol, phenolic resins, polyvinyl acetal resins, melamine resins, nitrocellulose, ethylene-acrylic acid copolymers, polyamide resins (e.g., nylon 6, nylon 66, nylon 610, copolymer nylon, and alkoxymethylated nylon), polyurethane, gelatin, polyvinyl pyrrolidone, polyvinylpyridine, and polyvinyl methyl ether.
It has also been proposed to form an intermediate layer using an organozirconium compound, e.g., a zirconium chelate compound or a zirconium alkoxide, or a silane coupling agent as disclosed in JP-A-59-223439 and JP-A-62-273549 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
What is aimed at by providing a resin subbing layer is to control volume resistance at such a low level that does not deteriorate electrophotographic characteristics by chiefly using a resin having a relatively large content of a polar group. Since volume resistance of a resin has a character of being dependent on ion conduction, it is considerably influenced by temperature and humidity. That is, when a photoreceptor is exposed to a low temperature and low humidity condition or a high temperature and high humidity condition, the resin layer has markedly increased resistance or markedly decreased resistance, respectively. An increased resistance may result in deterioration of electrophotographic characteristics of the photosensitive layer, while a decreased resistance may result in loss of expected functions of the subbing layer.
Thus, it is only part of the above-described disadvantages associated with a photoreceptor that could be overcome by proving a conventional resin layer. Susceptibility to environmental influences being taken into consideration, the effects of the resin layer are reduced by half. Therefore, the conventional resin layers have achieved only insufficient technical improvements.
Where an organozirconium compound, e.g., a zirconium chelate compound or a zirconium alkoxide, or a silane coupling agent is employed as a subbing layer, the above-described problem is considerably settled, and there is obtained an electrophotographic photoreceptor which has reduced dark decay and excellent chargeability, hardly undergoes a reduction in development contrast, has a particularly reduced residual potential, is less subject to variations of electrophotographic characteristics with environmental changes, and is excellent in durability. An electrophotographic photoreceptor of this kind hardly develops image defects, such as white pepper, black pepper, roughness, and pinholes.
However, the problem arising from use of the organozirconium compounds or silane coupling agents is that these compounds have poor film-forming properties and often cause cracks during drying after coating, which gives rise to another cause of image defects.